A chemical vapor deposition method (CVD) is a technique to precipitate, by use of a processing gas that does not react at room temperature, a reaction product in a film state on a surface of an object to be processed through vapor-phase growth in a high-temperature atmosphere, and this method is widely employed for such purposes as manufacture of semiconductor, and surface modification of metals and ceramics. Recently, the CVD, especially low-pressure plasma CVD, has been increasingly applied to surface modification of plastic containers, particularly to improvement of gas barrier properties.
The plasma CVD uses plasma for growth of thin film. In this method, basically, a gas including the processing gas is caused to discharge electricity by high-field electric energy under decompression, and a substance thus produced through dissociation and bonding is chemically reacted in gas phase or on the object to be processed, thereby depositing the substance on the object to be processed.
A plasma state is accomplished by glow discharge, corona discharge and arc discharge, and known methods of the glow discharge include, for example, a method utilizing direct current glow discharge, a method utilizing high-frequency glow discharge, and a method utilizing microwave discharge.
Among these methods, the method utilizing microwave discharge allows configuration of an apparatus to be significantly simplified. Moreover, when an internal surface of the plastic container is to be processed, a degree of decompression in the apparatus may be such that the microwave discharge is caused only in the plastic container. Therefore, it is not necessary to maintain a high vacuum entirely in the apparatus, leading to simple operation and superior productivity.
The following is a method disclosed as a microwave plasma processing method directed to the plastic containers. For example, a bottle is disposed in a cylindrical microwave confining chamber coaxially with a central axis of the microwave confining chamber. Then, air is exhausted simultaneously from spaces inside and outside the bottle, and the processing gas is put into the bottle for a predetermined processing time. Additionally, microwaves are introduced into the microwave confining chamber, and the microwaves in the microwave confining chamber are set in a TM resonance mode, and then plasma is ignited and maintained inside the bottle, thereby processing the bottle.
However, the above method has a problem in that a uniform thin film can not be formed on the container to be processed because the plasma state in the microwave confining chamber is unstable and strength distribution of plasma is formed in an axial direction of the confining chamber.
Another problem is that the processing period is long due to difficulty in causing plasma emission. Still another problem is that a nozzle to supply the processing gas is easily fouled.
In view of the foregoing problems, an object of the present invention is to provide a microwave plasma processing method capable of forming a uniform thin film on a surface of an object to be processed, and accomplishing the processing in a short time.